A way to combine a vertex shape with a pose?
Per hand Driver Approach,

Add a custom property (for example sake "prop") to the hand bones (assuming names "hand.L" and "hand.R") of your armature.  Use this property to drive your fist shape-keys.

Hunt down the hand bone as the variable target, make it a single property variable type,  with pose.bones["hand.R"]["prop"] the data_path.



Each bone custom property is saved with the pose.